1. Field of the Invention
The present invention relates generally to electronic circuit packages and in particular to an electronic circuit package including a lid covering an electronic circuit, the lid being attached to a solderable seal ring around the electronic circuit with a fluxless solder preform having an undesired oxide surface by use of a hydrogen atmosphere that, when heated, reacts with the undesired oxide on the solder and on any other metal surfaces thus reducing and removing the oxide layer and allowing the lid to be sealed to the substrate using solder.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Surface-mounted packages for electronic circuits are well known in the art. They generally include an electronic component, a plurality of leads, and a specially configured housing that encloses and protects the electronic component. The outer ends of the leads generally extend adjacent the lower edge of the housing for physical contact with corresponding solder pads on a printed circuit board during solder reflow.
When such electronic packages are required to be hermetically sealed the lids are generally welded to the base to form the hermetic seal and create the package. Thus in U.S. Pat. No. 4,737,742 a package is formed from a printed circuit board and a cover bonded to the board through a spacer that hermetically seals the surface of the board on which the SAW devices are installed. The cover consists of a ceramic plate. The spacer is an adhesive made from glass, resin, or the like.
In U.S. Pat. No. 4,639,631, metal areas on a base are electrostatically sealed to corresponding glass surfaces on a cover to create a hermetic seal for a surface acoustic wave device.
In an article in the 1985 Ultrasonic Symposium entitled "A Miniature Hybrid Circuit SAW Oscillator Using an All Quartz Packaged Resonator", by G. K. Montress et al., beginning with page 277, a miniature SAW resonator oscillator was disclosed in which the entire oscillator is contained within a volume of only 0.25 cubic inch. In that case, the SAW device was formed on a substrate and the matching quartz cover placed over and spaced from the substrate by a glass frit. Exposed bus bars have leads attached from them to the printed circuit board on which the device is placed.
In another article in the 1984 IEEE Journal on "Selected Areas in Communications",Volume SAC-2,No. 6,November 1984,beginning with page number 966, an article by Paul A. Dawson et al., entitled "An Undersea Fiber-Optic Regenerator Using an Integral-Substrate Package and Flip-Chip SAW Mounting" disclosed a novel method of mounting SAW filters and designated the mounting as a "flip-chip" that used a thick-film substrate as the base of the package. A lid was then laser welded to the base to form a hermetic seal and create the package.
In U.S. Pat. No. 5,111,277, a package is formed and sealed with a metallic cover preferably Kovar or another metal having a coefficient of thermal expansion approximately equal to that of the ceramic frame. The metallic cover is bonded to a seal ring manufactured from low expansion metals such as Kovar by a method such as seam welding. In seam welding, arc or resistance welding produces a series of overlapping spot welds which form a continuous, hermetic seal.
Special problems occur in attempting to design a hermetically sealed package for a surface acoustic wave device. Various types of resonators with glass containers have been suggested such as, for example, set forth in U.S. Pat. No. 4,639,631. The packages disclosed comprise two halves which are made of glass, or glass and ceramic, and have a recess in one half for receiving a quartz resonator. Such glass packages are bonded together about their periphery by means of a eutectic metal, such as solder. A hermetic seal is formed about the periphery of such a package due to the melting of the deposited eutectic material. However, depositing a eutectic material having a relatively low melting point about the periphery of each half of the glass package may require an intermediate layer of a different material since the desired eutectic material may not directly adhere to the glass.
A solder with a flux paste has been used in the past to package some electronic circuits in a hermetically sealed package. However, such combination of solder and flux cannot be used in forming hermetically sealed packages for surface acoustic wave devices that are sensitive to mass loading, inasmuch as the flux, upon the melting of the solder, can engage the electrode fingers of the surface acoustic wave devices and thus change their electrical operating characteristics. Thus, many other methods have been attempted to be used as described earlier to form hermetically sealed packages for surface acoustic wave devices.
A fluxless solder cannot be used to seal metallic packages together if it has an oxide coating on the surface of the solder and/or the packages. It is also known that placing such fluxless solder in a hydrogen environment at the proper temperature will cause the hydrogen to react with the oxygen in the undesired oxide coating to produce moisture and leaving the fluxless solder and/or package with a clean surface for bonding.
Further, the use of prior art welding of metal lids to a substrate requires a metal lid having an upper portion forming a lip for welding that increases the thickness of the lid. Also, a metallic lid containing gold can be sealed to a gold seal ring but obviously the resulting product is very expensive.
It would be advantageous to have a hermetically sealed package for a surface acoustic wave device that would be economical to produce through the use of a fluxless solder having an undesired oxide surface, that can be manufactured in volume production, that does not impact the electrical operating characteristics of the SAW device, is robust but economical to form and can absorb a great deal of shock, and provides a Faraday shield for the surface acoustic wave device.